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Power Supply Basics:
Power supplies are an essential part of any piece of electronic equipment, and while it's easy to take them for granted, it's surprising how complicated power supplies can be. This article discusses the basics of power supplies with an audio slant - it's particularly appropriate for people experimenting with "chip-amps".
While the power supplies found in audio amplifiers might look simple, there is more to them than meets the eye and it's vitally important to get the design right. Consider this: all the energy delivered to your loudspeakers comes from the power supply - the power amplifier circuit is just like a "tap", controlling the flow of energy.
With chip-amp based power amplifiers, the power supply is more important than usual. When designing a discrete amplifier, you can build in a high level of power supply immunity - for example the critical input stage and voltage gain stages can be highly isolated from the noisy output stage with simple R-C filters. But this isn't an option with most chip-amps where the power supply pins are shared with the high and low current circuitry.
Power supply requirements:
The power supply must convert the available power source into the form required by the amplifier circuitry. In most cases, this power source will be the domestic mains power supply - 115 or 230 volts, depending on where you live. But sometimes batteries are used - these must be configured correctly, and a means of recharging them might be required.
Let's consider the issues:
Output voltage and current:
This is the basic starting point when designing the supply. A typical requirement for a low powered audio power amplifier might be around +/-30V at 3 amps. For an audio pre-amp, you might need +/-15 volts at 100mA. Or a logic circuit might require 5V at 500mA.
The output power of a supply is the product of the voltage and current - 180 watts (60V times 3A), 3 watts and 2.5 watts respectively for the three examples above.
The load requirements can differ over time - the audio power amplifier might only need the full 180 watts when being asked to play a very loud passage into "difficult" loudspeakers, and even then the demand peaks might only last for a few milliseconds (assuming it's a class B amplifier). For this reason you can, and commercial manufacturers do, take liberties when specifying the components. This means "save money" or, more cynically, "cut corners".
By contrast, the preamp and the logic circuit will have a fairly constant current demand. Variables might be relays or display backlights, but these are easy to predict.
Important fact: the diyer can gain a significant increase in long-term reliability by over-engineering the power supply, often at minimal extra cost.
Finally, it is important that the power supply can reliably deliver the required energy comfortably without sustaining damage!
Output noise and ripple:
The ideal power supply is a perfect DC voltage source. Inevitably, such power supplies don't exist, and a practical power supply will have an unwanted AC component superimposed on the DC voltage. Batteries can get pretty close to ideal in this regard.
For a mains power supply, this AC component will contain multiples of the basic mains frequency - eg, 50, 100, 150 and 200Hz or 100, 200 and 300Hz. Additionally, there will be some high-frequency noise, some of which might be random, the rest being related to the mains frequency.
How an amplifier responds to this depends entirely on the design. As stated above, op-amp based power amplifiers can be much more sensitive to power supply noise than conventional power amplifiers, so effort spent on the power supply is definitely worthwhile.
Load and line regulation:
The output voltage of a power supply will fall when a load is connected. This is because of the internal resistance present in a practical voltage source - this effect was mentioned when discussing op-amps.
Any voltage source can be modeled like this, and it's easy to see how the internal resistance combines with a load to form a potential divider. The fall in output voltage for a change in load current is called load regulation, and is normally expressed in percentage terms. For the sort of unregulated supply used in audio power amplifiers, this value can be quite high - a few percent, whereas for a typical regulated supply that you might find in a preamp, this value might be 0.1% or better.
Line regulation refers to the change in output voltage caused by a change in input supply voltage. Again, the typical values depend on the type of supply in use - an unregulated supply will be entirely dependant on the value of the mains supply, whereas well-designed regulated supplies are almost completely independent from the input voltage, until it falls below a certain threshold.
Luckily, few audio circuits critically depend on the absolute value of the supply rails - the levels of noise and ripple are generally the biggest concern. Conversely, other applications such as logic or microprocessor systems can be very specific about their supply voltage.
This simple overview of the key parameters should help to set the scene - to read more specific details about power supply parameters, have a look at my bench PSU project.
AC Power Supplies:
This section investigates mains power supplies for audio applications.
When dealing with power supplies, safety must be your top priority. This also applies equally to batteries - the energy contained in these can be surprisingly destructive.
It might be an obvious thing to say, but your amplifier must not kill anyone! Also it must not set fire to your house! Safety must never be compromised at any time, whatever the goal. There is no justification for producing dangerous equipment. Ever.
You can take a number of practical steps to ensure your projects will be safe. Here are some basic guidelines:
1. Double-insulate all mains connectors:
2. Metal cases must be connected to Earth:
2. Always use appropriate fuses:
3. Ensure any wires passing though holes in the chassis are protected:
4. Always use good quality components:
5. Finally, use common sense:
The incoming mains supply is presented to the transformer via a fuse and an optional (but recommended) power switch. Larger amplifiers might require an anti-surge device, but that's beyond the scope of this article. Other optional extras include mains filters or remote-control power-up schemes.
Again, I recommend using an IEC socket for the mains inlet. These take up slightly more space than a captive mains lead, but they neatly get around the issues of safely securing the incoming cable. From experience, you'll find that once you've had to untangle the mains lead from the mess of cables behind your hi-fi a few times, you'll wish that you'd followed my advice!
Also from experience, mains wiring will take more space than you might think. Cables should not be cramped, compressed or chaffed - any undue pressure could cause the connections to become unreliable over time. Connections should be mechanically sound so that they do not rely on solder for strength. Some safety standards state that connections carrying mains power should not be soldered - only crimp or screw-type methods are permitted. It is absolutely your responsibility to check this - apart from the risk of personal injury, your household insurance might be invalidated if your amplifier burns your house down!
I've already said it, but all mains terminals must be protected so that no-one can accidentally touch them.
Safety aside, the objective is to get the incoming mains voltage to the primary winding of the mains transformer. This diagram shows an example:
This represents the most basic setup - the only optional component is the mains switch. Again, the fuse is absolutely mandatory. In the UK, domestic mains plugs also have a fuse fitted, but these are only available in a limited number of values (typically 1, 3, 5 and 13 amps), and are intended to protect the mains cable only. In other parts of the world, the nearest fuse might be back at the house fusebox, and this is likely to be rated at 20 amps and upwards. Without an appropriate fuse, a fault could turn an amplifier into a house fire!
The fuse should come first - thus it protects the switch should some mechanical fault cause live and neutral to come into contact. Note that if you are using a panel-mounted fuseholder, there is a correct way to wire them up - the live feed from the mains input should connect to the end of the fuseholder rather than the side. Otherwise, it's possible to receive a shock when inserting a new fuse. Panel-mount fuseholders intended for this application are much deeper than usual to try to minimise the risk of accidental contact, but of course require more space in the enclosure. IEC mains inlets with integrated fuseholders are available and these are preferable because you cannot access the fuse when the IEC plug is inserted.
Should you be using a double-throw switch, wire it as shown - otherwise the unconnected terminals of the switch become live when the switch is in the "off" position. Fortunately, most mains switches tend to be single-throw, but if you are using a relay to switch the mains, this is important. If you are using a single-pole switch, it should switch the live (not neutral) side. In this instance, the order of the fuse and switch can perhaps be swapped, but I would still prefer the fuse first because there is a (very) slight risk of the switch suffering mechanical damage that causes a short-circuit to the chassis.
The safety earth connection needs special mention. This is absolutely essential for safety, and this connection must carry a large current in the event of a live-chassis short-circuit. Some safety standards recommend that this connection is made using a tag that is secured to the chassis using a fastener that is not responsible for supporting anything else, and I follow this practice without exception. Always use shake-proof or "star" washers to ensure a good contact.
Choosing the fuse rating is notoriously difficult - you need a value that is large enough to not suffer from "nuisance tripping", yet not so large that it doesn't fail during a genuine fault. In theory, you could work out the highest current that is likely to flow under worst-case circumstances, and fit the next largest size. However, the discharged smoothing capacitors will cause a surge at switch-on, also torroidal transformers cause a large surge when first powered - anything greater than around 250VA is likely to need some form of surge-protection to stop the fuse from crying "wolfe".
Fuses are available with different "ballistics" - in other words "anti-surge" examples fare better here than "quick-blow". A prefix or suffix identifies these - for example T1A is 1 amp, anti-surge.
Essential to reduce the mains voltage to a suitable level to power your circuitry, and to provide safety isolation. Never build a circuit to run directly from the mains!
There are two basic types - the conventional "frame" devices made using "EI" laminations, and ring-shaped "torroidal" models. The former are normally used when cost is more important than size and external radiation, whereas torroidal transformers are considered to be the de-facto option for high quality audio, due to their compact size and low magnetic flux leakage.
Some people prefer frame transformers, claiming they "sound" better. Comparing the raw specifications, torroidal transformer appear to be better in all regards so it's hard to see where this claim comes from. However there is one factor that is rarely discussed - torrodal transformers have a much better frequency response, but ironically that is a distinct drawback - more high-frequency mains-bourn noise can get through to the audio circuitry. Mains filters are a cost-effective solution to this problem.
The mains is applied to the primary winding, and the output is taken from the secondary winding(s). There are a couple of options for the primary side - you can purchase transformers with a single primary winding to match your local mains supply - 230V, for example. This results in simpler wiring, but should you move to a different part of the world, you would need to obtain a new transformer. Alternatively you can use a transformer with dual primary windings. These are typically rated at 115 volts each, and can be connected in parallel or series for 115V or 230V operation. Therefore, it's relatively easy to re-wire these should you emigrate. Also, you could purchase a special 115-230V selector switch - this is the best plan if you plan to export your products.
The secondaries also tend to be dual, but some transformers have a single secondary winding with a centre-tap. Creating the dual supply rails required for typical audio circuits is possible from either.
Transformers are sold by their VA rating - meaning volts times amps. Power is normally measured in Watts, and is also the product of volts and amps. So what is the difference between VA and Watts? Into a resistive load such as a bulb or heater, none whatsoever. The complication comes when you drive a reactive load - then there is a phase difference between the current and voltage. In this circumstance, the difference between VA and Watts is called the Power Factor. Perhaps the subject of a future article...
As mentioned in the introduction, you can take liberties with sizing transformers for audio power amplifiers, and the cost saving can be considerable for a manufacturer. Personally, I feel that this cost saving is a false economy for DIY audio - the price-per-VA falls as the transformer rating increases, making large transformers more attractive. Other factors, such as physical size and weight are likely to be the issues I'd consider first.
Rectification and smoothing:
The next stage is to convert the AC from the transformer into DC - this process is called rectification. AC current flows in differing directions, whereas DC current flows in one direction only. Therefore, to rectify AC, we need to use diodes - a component that acts like a one-way valve. This diagram shows some basic ideas:
Circuit A is sometimes used for low-power applications, and when current demands are a few mA, this circuit is just fine. But for more realistic loads, this circuit is inefficient because energy only flows for the positive half-cycle.
Circuit B uses a bridge rectifier to avoid this problem, albeit in return for a small increase in complexity. However, it is possible to obtain this configuration of four diodes in a single package, making practical realisations very simple indeed.
This diagram shows the waveforms resulting from the above circuits. In both cases, the AC input is shown in red, and the rectified DC is blue. The slight difference between the input and output is caused by the forward voltage drop of the diodes (around 0.6-0.7 volts - note that there are two diode drops for a bridge rectifier).
The output from both these circuits varies from 0 volts, up to almost the peak voltage of the incoming AC voltage. Put another way, the output contains a large AC component which needs to be removed. Something needs to "fill in the gaps", and that's where the smoothing capacitors come in...
The green trace shows the voltage across a capacitor connected to the output of each of the circuits. As you can see, energy is supplied by the smoothing capacitor when the AC voltage falls below the DC output. During the peaks, energy is supplied from the transformer via the rectifier - this energy recharges the smoothing capacitor.
The problems with half-wave rectification become apparent here - the capacitor is "topped up" less often, so the charge in the capacitor falls more. This means the ripple voltage is higher.
This diagram attempt to clarify things by zooming right in to examine a peak. As before, the green trace is the voltage across the smoothing capacitor, and here we see this voltage at both its minimum and maximum values. The difference between these values is the ripple voltage.
The conduction angle is defined as the time when energy flows from the transformer into the capacitor and load. It's normally expressed in degrees, remembering that one complete cycle of a sine wave has 360 degrees.
So, current is taken from the transformer, and therefore the mains supply, in chunks. Every cycle, a full-wave rectifier takes two "gulps" from the supply to recharge the smoothing capacitor. During this time, enough energy is taken to refill the capacitor such that it can supply energy during the remainder of the cycle, so it follows that the peak currents flowing during conduction time are much greater than the average current flowing in the load. So the bridge rectifier must be rated at a higher current than you might expect.
This Spice simulation shows that the charging current peaks are approximately 4 times larger than the average current flowing into the load. For full-wave rectification, this isn't a bad "rule of thumb" to bear in mind. It tells you that the wiring between the transformer, bridge rectifier and smoothing capacitor needs to be much thicker than you might think, given the average current drawn from the supply.
Lets consider an example power supply, with some typical values:
The AC voltage from the transformer is 12 volts. But note how the output voltage is higher - how is this possible? AC voltages are specified in RMS (Root Mean Squared). But rectifying circuits with smoothing capacitors produce a DC voltage that is close the peak value of the AC input, which is bigger than the RMS value by a factor of 1.414 (the square-root of 2).
The exact DC voltage is hard to predict because it depends on many variables, including the load current, the ripple voltage, the diodes used, the transformer, etc. So, multiplying the AC voltage by root-2, and knocking off a volt or two will get you close enough for practical purposes. Should your application need a more precise voltage, you need further circuitry to regulate the supply.
This simple supply is often called an Unregulated Supply. As shown here, it produces a single DC voltage from one transformer secondary. However, audio applications frequently require a symmetrical supply - here are a couple of ways to do this:
The first example is widely used to produce split supplies from a centre-tapped secondary windings although, as mentioned above, most transformers have dual secondaries, which can be connected in series to form a centre tap.
The second example looks like two single supplies that have simply been connected together, and this is basically the case. This uses more components, and losses are higher because of the extra diodes, but this method offers easier ground management. In other words, it's easier to keep the large charging current pulses away from the system ground. Ensuring a clean, noise-free ground is essential in any audio power supply.
Summary and conclusions:
This page has introduced a lot of the basics - including safety, mains wiring, transformers, rectification and smoothing.
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